1. Field of the Invention
The present invention relates to a conductive paste which is printed on a glass substrate and baked to form defogging heat wires for automobile windows.
2. Description of the Related Art
Automobile windows are provided on the surface thereof with conductive wires for preventing the window from fogging and which are formed by printing and baking a conductive paste. The conductive wires heat up when energized and, thus, prevent the automobile window from fogging. Also, the regions of the window where the conductive wires are disposed have a dark color so as to be nice to look at when viewed from the outside of the automobile.
Such a conductive paste generally contains an additive for darkening the external appearance of the window glass. As this additive, V, Mn, Fe, Co and their oxides (Japanese Unexamined Patent Application Publication No. 5-290623) and Cr, Rh and their oxides (Japanese Unexamined Patent Application Publication No. 9-92028) are known.
However, these additives have problems of (1) making the color of the conductive wire regions of the window nonuniform due to the formation of a low, melting point salt resulting from a reaction with part of the silver contained in the conductive paste; (2) degrading solder wettability due to an oxide coming to the surface of conductive wires together with glass after baking, and thus reducing bonding strength of lead wires; (3) being likely to vary the valence of transition metals during baking and, thus, to produce harmful substances; and (4) increasing cost when Rh is used.